In order to reduce the volume, weight, and noise to improve transient response and achieve high power density, high frequency can be applied for switching converters. To decrease the switching losses for high frequency applications, a soft switching technique may be utilized. By resonant controlling and driving for the power switches, both the voltage and current of the power switches of the switching converter may vary in sinusoidal or quasi-sinusoidal waveform. When the voltage crosses zero, the power switch can be turned on, and when the current crosses zero, the power switch can be turned off. By such operation, the switching losses can be lowered to essentially zero.
The flyback converter may be applied widely for lower power applications. The amplitude of switching frequency may be limited due to capacitive conduction loss of the power switch. There are many ways to achieve soft switching, most of which are unavailable for applications with higher input voltage because the power switch may bear much higher voltage stress. Resonance can occur between the intrinsic body capacitor of the power switch and the primary inductor of the transformer after the inductor current decreases to zero when the inductor current is operated in a discontinuous conduction mode.